1. Field of the Invention
The present invention relates to a color image processing apparatus for use in a color scanner, a printer, a facsimile machine or the like, and more particularly to a color image processing apparatus using a threshold value of a digital halftone screen which prevents the occurrence of moire and generates a regular rosette pattern when a plurality of dot images having different screen angles in accordance with colors are superimposed on each other.
2. Description of the Related Art
Hitherto, a color image processing apparatus for generating a dot image uses a halftone screen set having screen angles of .+-.15 and 45 degrees to produce image signals of three color; a C (cyan), an M (magenta) and a Y (yellow) by dot-processing.
According to the "PostScript Screening" by Peter Frink published by "Adobe Systems Incorporated" in 1992, that three plates having identical dot intervals are superimposed on each other at the interval of 30 degrees satisfies the condition under which the finest moire occurs. When the dot processing is digitally made, a screen angle is set to +15 degrees relative to magenta, -15 degrees relative to cyan and 45 degrees relative to yellow, respectively. In order to generate image signals of dot images having these screen angles, a unit area equal to an area which can be occupied by one dot is determined, and threshold values corresponding to a plurality of pixels equal to or more than the number of gradations in the unit area are set. Such threshold values constitute threshold values in a digital halftone screen. Finally, the threshold values in the digital halftone screen are compared with color image signals obtained by scanning color original images, thereby to output its result in the form of digital values, that is, 1 and 0. When recording, a pixel corresponding to the digital value of 1 is recorded in the unit area thereby to record the dot image.
The digital halftone screen has a structure in which dot cells 301 constituting a minimum unit of the threshold value are cyclically arranged as shown in FIG. 1, and represented by a periodic function of the following formula (1). EQU h(x-p, y-q)=h(x, y) EQU h(x-q, y+p)=h(x, y) (1)
Cyclic constants (p, q) in the formula (1) determines the screen angle and the number of gradations of the halftone screen. That is, the screen angle .theta. is determined in accordance with formula (2) whereas the number N of gradations is determined in accordance with formula (3) EQU .theta.=tan.sup.-1 (q/p) (2) EQU N=p.sup.2 +q.sup.2 +1 (3)
x, y, p and q in formula (1) to (3) must be integers, however, when the screen angle .theta. has accurately .+-.15 degrees, all of x, y, p and q are not the integers.
For example, the halftone screen having fifty gradations at the screen angle of 15 degrees cannot be constituted because of a limitation that both of formulas (2) and (3) are satisfied and also the cyclic constants (p, q) are integers. In this case, a pair of cyclic constants extremely capable of approximating to ideal values is unavoidably selected. For example, if p=7 and q=2 are approximate values thereof, the halftone screen having the screen angle of 15.9 degrees and the number of gradations of 54 can be obtained.
However, in the conventional halftone screen, screen ruling representing the number of dot boundary lines (the number of dots) per inch, are different from each other between a screen having the screen angle approximating to .+-.15 degrees and a screen having the screen angle approximating to 45 degrees.
Therefore, in the case where the screens of C, M, and Y are superimposed on each other for recording, there arises a defect to occur moire which cannot be forecasted.